1. Field of the Invention
The present invention relates to a gas separation apparatus and gas separation method for separating specific gases from a gas to be treated which contains a plurality of specific gases.
2. Description of the Related Art
Conventionally, various gases are used in semiconductor manufacturing processes depending on the particular process adopted. For example, perfluoro compound (PFC) gas which is a mixture containing fluorine compounds such as CF4. NF3, C2F6, C3F8, SF6, and CHF3 is used as a reaction gas at the dry etching process or at the cleaning process for a thin film forming device. In these processes, a discharge gas is produced which contains the PFC gas.
Because these discharge gases such as PFC promote global warming and thus cannot be discharged out of the line without further processing, various methods are employed for treating these gases. The treating methods include (i) decomposition in which the PFC gas is decomposed by combustion, catalyst heating, or plasma decomposition; (ii) membrane separation in which these materials are separated by a membrane; (iii) distillation separation by subzero cooling in which the separation is achieved taking advantage of the difference in the boiling points of the gases; and (iv) chromatographic separation in which separation is achieved by taking advantage of the difference in time for passing through a chromatographic column.
However, in the decomposition method (i), there are shortcomings in that a complete decomposition is difficult and the gas cannot be recovered for reuse because the gas is decomposed and discharged. In the membrane separation (ii), although nitrogen in the discharge gas can be removed, the separation between PFC gases is difficult. In the subzero cooling separation method (iii), separation between gases which only have a small difference in the boiling points, such, for example, as CF4 and NF3 with the difference being only 1xc2x0 C., is difficult. In a conventional PFC recovery device employing the subzero cooling separation method, when recovering a mixture gas of CF4 and NF3, NF3 must first be decomposed and the remaining CF4 alone is then recovered. However, the trouble with this method is that NF3 which is the most expensive gas among the PFC gases cannot be recovered. In the chromatographic separation (iv), there is a problem that when three or more PFC gases are present, these gases cannot simultaneously be separated and recovered.
The present invention has been conceived to overcome the shortcomings of the conventional gas treatment methods mentioned above, and one object of the present invention is to provide a gas separation device and gas separation method capable of separating a mixture gas having a plurality of constituents, in particular, three or more constituents, inexpensively and with high purity.
According to the present invention, a mixture gas to be treated containing a plurality of gas constituents is first separated by distillation separation into a plurality of gas constituent groups, each of which group has similar boiling points, and then the specific gases within each of separated gas constituent groups are separated by chromatographic separation. In this manner, a mixture gas having a plurality of constituents, in particular, three constituents or more, can be separated inexpensively and with high purity.
For example, when a mixture gas having a plurality of gas constituents includes CF4, NF3, C2F6, and CHF3, and nitrogen is an additional gas constituent, the gases can be separated by distillation separation into a first gas group which includes CF4 (having a boiling point of xe2x88x92128xc2x0C.) and NF3 (having a boiling point of xe2x88x92128.8xc2x0C.), a second gas group which includes C2F6 (having a boiling point of xe2x88x9278xc2x0C.) and CHF3 (having a boiling point of xe2x88x9282.2xc2x0C.), and a third gas group which includes nitrogen (having a boiling point of xe2x88x9295xc2x0C.). Then, by chromatographic separation, the mixture gas of the first gas group can be separated into CF4 and NF3. The mixture gas of the second gas group can similarly be separated by chromatographic separation into C2F6 and CHF3. The separated gases of CF4, NF3, C2F6, and CHF3 can respectively be recovered for reuse.